1. Field of the Invention
The present invention relates to a vibration motor and, more particularly, to a vibration motor that is preferably used for incoming call notification and the like in a mobile terminal device.
2. Description of the Related Art
Among mobile terminal devices such as mobile phones and the like, there is a function of generating vibration to notify a user of an incoming call, in addition to a function of generating a ringtone to notify the user of the incoming call. The vibration for notifying an incoming call on a mobile terminal device is generated by a vibration motor that is provided to the mobile terminal device. A structure for generating vibration by a weight that is eccentric relative to and rotates around the central axis is one of the vibration-generating structures provided in vibration motors.
A vibration motor that is proposed in Japanese Unexamined Patent Application Publication No. 2005-117849 is an example of vibration motors in which an eccentric weight rotates about the central axis to generate vibration. This vibration motor has a stator with an embedded drive circuit, and a rotor rotatably mounted thereto with a gap between the stator and rotor in an axial direction. The rotor consists of a magnetic stainless steel rotor yoke, an axial gap type magnet, and a sintered oil-less flange bearing. The axial magnetic stainless rotor yoke is disposed on the axial gap type magnet. The sintered oil-less bearing is provided to an inner diameter portion of the magnet. The rotor yoke consists of a flat portion receiving a magnetic field of the magnet, an outer diameter side skirt portion fixedly holding the arcuate eccentric weight, and an inner diameter side skirt portion rotatably supporting the axis. A tongue piece is integrally formed with and protrudes horizontally outwardly from the outer diameter skirt portion. Also, the eccentric weight is fixed to the tongue piece by soldering or adhesion. Further, the magnet is mounted by adhesion, or by spot welding and the like in case a sintered rare earth metal magnet is used.
However, in case of soldering or bonding the eccentric weight by adhesion to a member constituting the rotor such as the rotor yoke and the like, as shown in the vibration motor suggested in Japanese Unexamined Patent Application Publication No. 2005-117849, the number of processes increases in accordance with the proportion of the soldering or adhering process. Also, it is required to make a design in consideration of the tolerances accumulated when soldering or adhering the eccentric weight. Even if the design was made in consideration of the accumulated tolerances, it is difficult to make the actually assembled vibration motor as originally designed so that the desired performance may not be provided. Moreover, in case of soldering or bonding by an adhesive, the applied lead (alloy) or adhesive may be pressed out from certain places and cause some defects in manufacturing.